The present invention pertains to a heterostructure acoustic charge transport (HACT) devices and more particularly to an arrangement for improving device performance by reducing the magnitude of the surface acoustic wave (SAW) required for effective HACT device operation.
An HACT device employs a powerful ultra high frequency (UHF) SAW propagating on the top, highly polished surface of a wafer of piezoelectric semiconductor material, usually gallium arsenide (GaAs), to bunch mobile charge carriers in extrema of the SAW electrical potential and to then transport these discrete charge packets at the speed of sound through semiconductor material, as is described in detail in U.S. Pat. No. 4,893,161, entitled "Quantum-Well Acoustic Charge Transport Device," issued to William J. Tanski, Sears W. Merritt and Robert N. Sacks. Further information regarding ACT and HACT devices is found in U.S. Pat. No. 4,980,596, entitled "Acoustic Charge Transport Device Having Direct Optical Input", issued to R. N. Sacks et al.; U.S. Pat. No. 4,926,083, entitled Optically Modulated Acoustic Charge Transport Device", issued to S. W. Merritt et al.; U.S. Pat. No. 4,884,001, entitled "Monolithic Electro-Acoustic Device Having An Acoustic Charge Transport Device Integrated With A Transistor", issued to R. N. Sacks et al.; and U.S. Pat. No. 4,633,285, entitled "Acoustic Charge Transport Device And Method" issued to B. J. Hunsinger et al. ; the above-noted patents are incorporated herein by reference. The SAW thus functions as an acoustic clocking signal, similarly to clocking signals in a conventional charge-coupled device (CCD), but without need for complex interconnections which CCDs require. SAW transducer/channel width design considerations are addressed in "A Synopsis of Surface Acoustic Wave Propagation on {100}-Cut &lt;110&gt;-Propagating Gallium Arsenide" by W. D. Hunt et al., (J. Appl.Phys. 69(4), pp. 1936-1941, 15 Feb. 1991), which is incorporated herein by reference.
CCDs, bucket brigade devices and related memory devices have the disadvantages that great fabrication complexity is necessary in order to provide the polyphase clocking signals required by such devices. Other disadvantages of such devices include limited practical clocking speed and substantial dark currents. The capacitive nature of the transfer electrodes to which the clocking signals are applied exacerbates difficulties involved in attempting to increase clocking speeds because these types of charge transfer devices tend to operate best when driven by clocking signals having sharp transitions, e.g., square waves, which are difficult to supply to capacitive loads such as clocking electrodes, especially at high frequencies.
Another class of device which has been experimentally demonstrated at low clocking frequencies (e.g., less than 100 MHz) but which has not shown transfer efficiencies of practical value even at these low clocking frequencies are based on metal-insulator-semiconductor (MIS) technology wherein a layer of strongly piezoelectric material is applied over a layer of native oxide to a semiconductor material such as silicon. These devices operate by first establishing a channel region at the semiconductor-oxide interface by means of an electrical bias which gives rise to an inversion layer (e.g., a two-dimensional minority carrier sheet) which is then subsequently bunched and synchronously transported by a SAW (or other acoustic wave) launched from a SAW transducer which is in proximity to the piezoelectric layer. Such devices have consistently had serious problems in practice including high surface state densities at the semiconductor-oxide interface, giving rise to charge trapping effects and poor charge transfer efficiency even at low clocking frequencies, high acoustic propagation losses (limiting practical channel lengths and hence storage times), incompatibility with Schottky electrode sensing techniques for non-destructive charge sensing, substantial dark currents due to the high electrical fields necessary to provide inversion layers and poor input-output isolation due to the necessary presence of a conductive layer immediately beneath the charge-carrying inversion layer (inversion layers can only be formed by biasing a conductive layer). A particular problem has been that most dielectric films provide high acoustic propagation losses. For these and other reasons related to device processing problems, acoustically clocked MIS structures have never proven practical as charge transfer devices. Examples of concepts for such devices are briefly mentioned in passing in Boyle et al., U.S. Pat. No. 3,858,232, issued Dec. 31, 1974 and in more detail in Mikoshiba et al., U.S. Pat. No. 4,799,244, issued Jan. 17, 1989. Experimental performance of such devices is discussed in Tsubouchi et al., "Charge Transfer by Surface Acoustic Waves on Monolithic MIS Structure", 1978 IEEE Ultrasonics Symposium Proceedings, pp. 20 through 24 while theoretical predictions of performance absent charge trapping are presented in "Modelling of Charge Transfer by Surface Acoustic Waves in a Monolithic Metal/ZnO/SiO2/Si System" by F. Augustine et al., IEEE Transactions on Electron Devices, ED-29, No. 12, Dec. 1982, pp. 1876 through 1883. In "Fabrication-Related Effects in Metal--ZnO--SiO2--Si Structures", Cornell et al., Applied Physics Letters, Vol. 31, No. 9, 1 Nov. 1977, pp. 560 through 562, provides conclusive experimental evidence that the sputtering process used to deposit ZnO causes surface state problems which severely limit the performance of acoustically clocked devices using such structures. The above-noted patents and articles are hereby incorporated by reference.
The very weak piezoelectricity of GaAs (k.sup.2 =7.4.times.10.sup.-4 for the Rayleigh mode on 100!-cut, &lt;110&gt;-propagating GaAs) dictates that the great majority of energy in the SAW is manifested as mechanical energy and only a small portion of the total energy is manifested through the electrical potential which accompanies the SAW. It is this electrical component of total SAW energy which bunches charge carriers forming distinct packets and transporting these packets, representing the input signal, through the HACT device. Accordingly, present day ACT and HACT devices require large (about one Watt) acoustic power levels in order to realize the voltage required (about one Volt) to effect coherent charge packet transport within the HACT channel, synchronous with the SAW clock signal.
What is needed is a device architecture providing high charge transfer efficiency, long charge storage times, good frequency response, low clocking signal power requirements and low dark current.
Therefore, it is an advantage of the present invention to provide an HACT device which includes a greatly reduced acoustic power requirement for achieving coherent, synchronous charge transport.